Optoelectronic devices, for example semiconductor light sources, are devices in which an optical input produces an electrical output, or in which electrical stimulation produces visible, infrared or ultraviolet output. To form a semiconductor light source, such as light-emitting diodes (“LEDs”) and laser diodes (“LDs”), for example, a multilayer structure is fabricated which comprises a substrate base as well as an active region including an n-type semiconductor layer electrically connected to a p-type semiconductor layer. The active region often comprises one or more quantum wells sandwiched between thicker cladding layers.
One useful type of semiconductor light source operates in the ultraviolet (“UV”) range. The wavelength of the light emitted depends on the band gap energy of the materials forming the p-n junction. III-nitride based devices are capable of achieving shorter wavelengths in the ultraviolet range. There exists a need for stable and long-lasting optoelectronic devices operating in the UV range, and more particularly, in the deep-UV range. The ability to provide UV-emitting devices featuring stable light output and long lifetimes would simplify system design and lower costs. However, commercially available devices operating in the UV range, particularly in the deep-UV range, have not reached the level of efficiency and technological maturity of the visible light-emitting devices.
Sapphire substrates are typically used to fabricate III-nitride based light-emitting devices. It is possible to obtain relatively low-cost III-nitride semiconductor devices by using highly transparent sapphire as a substrate; however, with poor reliability (very low lifetime). The reason is due to the incompatibility between the lattice constant of the sapphire substrate and the fabricated device layers, a large density of dislocations of approximately 109-1010 cm−2 is typically in the device structure. At the interface between the substrate and the device structure, the dislocation density will even be orders higher. The defect density in III-nitride based devices mainly refers to dislocation density and associated point defects. In the case of GaAs based LED and LD devices, for example, dislocations are the primary failure reason.
This elevated defect density negatively impacts the light emission efficiency and the lifetime of the device. Commercially available UVB (wavelength range of approximately 315-280 nm) and UVC (wavelength range of approximately 280-100 nm) optoelectronic devices (deep-UV or “DUV” LEDs) have short lifetimes, frequently as low as only tens to hundreds of hours, due to this high defect density resulting from the incompatibility of the substrate and the fabricated device layers.
There are some experimental reports of UV-emitting LEDs formed using aluminum nitride (AlN) substrates. However, the heretofore published performance information for such LED structures indicates a drop in output power to 80% in about 300 hours (i.e., an L80 of about 300 hrs) at an injection current of 150 mA. See Grandusky et al. (2010) Performance and reliability of ultraviolet-C pseudomorphic light emitting diodes on bulk AlN substrates, Phys. Status Solidi C, 7: 2199-2201, the entire disclosure of which is hereby incorporated by reference. If the L80 lifetime for an LED is very low, systems must be designed with excessive output power at the beginning of life so that the device remains within the minimum specifications at the end of its usable life.
In sum, considerable efforts have been devoted to producing UV LEDs on sapphire substrates, but the natures of the heteroepitaxy sets a high defect density that cannot be further reduced. Although it has been suggested that UV LEDs can be prepared from AlN substrates, such LED structures having high performance characteristics have not been demonstrated. Accordingly, there remains a need in the art for high performance, low defect density optoelectronic devices that emit light in the UV range.